1. Field of the Invention
This invention relates to color television
This invention relates to color television receivers, and more particularly to apparatus for controlling the operation of a color cathode ray tube (CRT) therein.
2. Description of the Prior Art
In color television receivers of the type employing a multi-gun color CRT, such as the well-known three-gun, shadow mask color CRT, a plurality of operating controls for the CRT ae conventionally provided. A number of these controls have no direct counterparts in the typical black and white television receiver, employing the usual single-gun black and white CRT; the additional controls are, of course, necessitated by the additional complexity of a multi-gun, multi-phosphor reproducer as compared with a single-gun, single-phosphor CRT.
To provide optimum operation of the color CRT, the controls associated with the color CRT are adjusted in accordance with a procedure, conventionally referred to as a color CRT set-up procedure, in order to correct for a number of variations inherently resulting from the multi-gun, multi-phosphor character of the reproducer. Such variations include differences in the cutoff potentials of the respective guns, differences in the cathode emission of the respective guns, and differences in the efficiencies of the respective phosphors. A goal of the set-up procedure may be viewed as obtaining the brightest picture possible, while maintaining proper "tracking" at all brightness levels. That is, it is desired that the color CRT reproduce white information with the proper color temperature at all brightness levels between maximum white and minimum white, with the color CRT being capable of reproducing information which represents the maximum white level at the highest achievable level of brightness. The set-up procedure associated with a receiver having operating controls as enumerated above requires a succession of interrelated adjustments of the brightness control and the various background and screen controls. The procedure is somewhat tedious, the interplay between the various controls requiring considerable ingenuity and patience to arrive at the precisely proper conditions. Adjustments of the background controls often require retouching of the screen controls, and vice versa.
A color television receiver representative of the prior art typically includes the following operating adjustments in association with the color CRT: means for adjusting the three separate guns to a current level near cutoff to establish a low brightness white color balance, and drive controls to achieve color balance at high brightness levels. The low level adjustment may be accomplished either by adjustment of the screen grid voltages of at least two of the three electron guns with a fixed bias between the cathodes and control grids, or by adjustment of the control grid to cathode bias of at least two electron guns while maintaining a fixed voltage on all three screen grids. The drive controls adjust the video gain to achieve black and white tracking in high brightness areas of the picture under dynamic conditions. The low level, sometimes referred to as "background adjustments", are usually performed with no video signal present and with the vertical deflection disabled to obtain a single horizontal line. The drive adjustments are then performed with normal video and deflection, but with the color disabled or turned off.
To achieve proper tracking normally requires at least four controls, two for the low level adjustment and two for drive control. Six controls frequently are used to achieve the best overall performance, i.e., maximum drive capability wtih optimum focus for all three electron guns.
In certain types of tubes employing the in-line slot mask, e.g. RCA 19VEJP22, all three screen grids are connected together internally and only one screen grid voltage adjustment is possible. In television receivers employing this type of tube, at least five controls are typically employed, consisting of at least two for background adjustment, two for drive adjustment and one for screen control. Up to seven controls may be used.
In accordance with present United States standards governing color television transmissions, luminance information, representing elemental brightness variations in a televised image, is transmitted on an amplitude-modulated main carrier component and chrominance information, representing color hue and saturation variations, is transmitted on a phase- and amplitude-modulated 3.58 MHz subcarrier constituent. Demodulation of the luminance component is generally accomplished by means of a conventional AM video detector, and results in composite video-frequency luminance signal having a bandwidth of approximately 4 MHz. Demodulation of the chrominance component requires in addition a synchronous detector, and results in three color-difference signals, commonly designated R-Y, G-Y and B-Y, which represent the difference between the respective primary colors and the transmitted luminance signal.
To control the tri-gun, tri-color shadow-mask-type CRT in almost universal use today, it is necessary to combine, or matrix, the three color-difference signals with the luminance signal to form color-control signals of the form R, G and B. While this may be done internally within the image reproducer by applying the signals at a sufficient amplitude directly to respective control elements of the tube, it is more efficient to instead matrix the color-difference singals with the luminance or Y-signal at a lower level externally to the color CRT and then amplify the resulting R, G and B signals to a level suitable for application to the color CRT.
An amplifier stage appropriate for this purpose, which may comprise a trio of individual amplifiers, one for each primary color, must necessarily meet certain functional requirements. For one, such a luminance-chrominance matrix amplifier stage must provide direct-current coupling between the luminance and color-difference signal sources and the color CRT to insure faithful reproduction. It must establish a reference voltage to which the color CRT can be set up or adjusted for cut-off, and must allow for individual adjustment of the amplitudes of the color-control signals applied to each gun to compensate for varying gun efficiencies without affecting either the reference voltage or the direct-current coupling. Furthermore, this stage may include suitable peaking circuitry for equalizing the higher-frequency video components with respect to the lower-frequency chrominance components of the composite video signal.
Referring now to FIG. 1, there is shown a prior art arrangement of a chrominance matrix amplifier, wherein the luminance signal as derived from a television luminance detector and luminance amplifier (not shown in FIG. 1), is added discretely by a separate amplifying element, typically a transistor, with each of the color-difference signals as derived from a chrominance demodulator (not shown in FIG. 1). The illustrated chrominance matrix amplifier 15' includes three distinct amplifying elements in the form of transistors Q.sub.B ', Q.sub.G ' and Q.sub.R ' for separately amplifying the color-difference signals B-Y, G-Y and R-Y, respectively. As illustrated, the suffixes B, G and R are used throughout this description to indicate elements related, respectively, with the processing or display of the blue, green and red color signals upon a cathode ray tube (CRT) 20'. In particular, the luminance signal is applied via an adjustable resistor R.sub.21B ' and resistor R.sub.20B ' to the emitter of transistor Q.sub.B '. In a similar fashion, the luminance signal is applied through variable resistors R.sub.21 ' and resistors R.sub.20 ' to the emitters of each of transistors Q.sub.G ' and Q.sub.R '. Effectively, the gain of each of the aforementioned transistors Q' is controlled by the adjustment of its connected resistor R.sub.21 '. In turn, the output from each of the aforementioned transistors Q' is derived from its collector and is applied to a corresponding cathode element 19' of the color CRT 20'. For example, the collector of transistor Q.sub.B ' is connected to the cathode element 19.sub.B ' of the blue electron gun.
In FIG. 1, the color CRT 20' is an in-line color CRT wherein the G.sub.1 elements associated with each of the three electron guns are internally connected and, in similar fashion, the G.sub.2 elements of the three color electron guns are internally connected. The G.sub.1 and G.sub.2 electrodes serve to accelerate and to focus the three electron beams onto a phosphorescent screen of the CRT 20'. Because of the common connection of each of the G.sub.2 electrodes, it is impossible to establish individual gun cutoff bias by separate control. Instead, it is necessary to adjust the bias as applied to each of the individual cathode elements 19.sub.B ', 19.sub.G ' and 19.sub.R '. As indicated above, one of the electron guns will require a greater bias than the remaining two electron guns; this one electron gun requiring greater bias is referred to as the dominant electron gun.
In a typical set-up procedure of the television set including the chrominance matrix amplifier 15' of FIG. 1, the resistors R.sub.21 ' are set to minimum. This provides maximum gain for each stage. The amplitudes of the color-difference signals are reduced to zero by means of the color control, e.g. a variable potentiometer within the chroma demodulator or the color matrixing circuit, and the luminance signal is removed by opening the common emitter drive line connected to the Y input. This may be accomplished by means of a switch connected in-series with this line or a removable plug-in jumper connector. The emitter currents of the transistors Q.sub.B ', Q.sub.G ' and Q.sub.R ' are now established by the transistors Q.sub.BB ', Q.sub.GB ' and Q.sub.RB ' which are connected as constant current sources in which the collector currents are established by the positive bias voltage applied to their respective bases as determined by the voltage source V and the setting of the respective potentiometers R.sub.23 '. The three potentiometers R.sub.23 ' then are set to minimum (in this case, zero). The collector voltages of all three amplifier transistors Q.sub.B ', Q.sub.G ' and Q.sub.R ' now are maximum, providing maximum bias on the three CRT cathodes 19'. Next, the voltage applied to the commonly-connected G.sub.2 element then is increased until a color appears upon the display screen of the CRT 20'. The particular color displayed thereon identifies the dominant electron gun of the CRT 25. Next, the potentiometers R.sub.23 ' of the remaining two electron guns are increased to obtain the desired low-brightness, gray-scale balance, i.e., the aforementioned potentiometers R.sub.23 ' are adjusted until a gray or low-brightness white raster (or line, if the vertical deflection has been removed) appears upon the display screen of the CRT 20'. Next, the voltage V.sub.G2 as applied to the commonly-connected G.sub.2 elements, is decreased so that the image is extinguished. In establishing the proper bias for the cathode elements 19', it is desired to maintain the maximum voltage difference between that applied to the cathode elements 19' and that applied to the commonly-connected G.sub.2 elements to maintain the best-possible focus of the electron beams onto the faceplate of the CRT 20'.
To establish the gain or drive of each of the driver transistors Q', there is included in the emitter circuit of each such driver transistor Q' the variable resistor R.sub.21 ' and fixed resistor R.sub.20 '. In particular, the luminance signal is applied via the variable resistor R.sub.21B ' and the fixed resistor R.sub.20B ' to the emitter of the blue driver transistor Q.sub.B '. By adjusting the impedance of the variable resistor R.sub.21B ', the gain of the blue driver transistor Q.sub.B ' is controlled correspondingly. After adjusting the bias currents as described, by means of potentiometers R.sub.23 ', the luminance signal is reconnected and the brightness and contrast controls are set for a normal picture. The picture typically will assume some color tint. The gains or drives of two stages are reduced as required, by increasing their respective resistors R.sub.21 ', until a satisfactory black and white picture is obtained. Color is then increased to normal level. Thus, three separate adjustments were required to set the gain of the corresponding driver transistors Q'. In addition, three further adjustments were necessary to set the bias of the driver transistors, i.e., setting the resistors R.sub.23 ' associated with the bias transistors. Noting the interrelationship between the setting of the cathode bias and the setting of the gains of the driver transistors, the setting-up of a color television including such a chrominance matrix amplifier becomes quite complex and tedious, requiring many readjustments.
Further, the prior art has attempted to reduce the complexity of the adjustments of cathode bias and driver amplifier gain within color television receivers, an example of which is U.S. Pat. No. 3,737,562, wherein a driver transistor amplifies one of the blue, green or red signals before applying it to the cathode element of a CRT. In particular, a variable resistor is connected in its emitter circuit with a constant reference potential source, such as a zener diode, the combination of the series-connected zener diode and variable resistor being bypassed by a further resistor. The noted patent suggests that at the point when current is established through the aforementioned resistors so that the potential at the point of interconnection of the bypassing resistor and the variable resistor equals the voltage established at the point of interconnection between the zener diode and the variable resistor, the potential as applied to the G.sub.2 electrode of the color CRT is adjusted to establish the black level of that electron gun. In the above-noted patent, it is contemplated that there are separate controls for establishing the bias applied to each of the distinct G.sub.2 electrodes, as opposed to an in-line color CRT wherein the G.sub.2 electrodes are interconnected with each other. The noted variable potentiometer controls the gain of its amplifying transistor and it is further suggested that such a variable resistor be interconnected in the emitter circuits of two amplifying transistors in the form of a chrominance matrix control whereby the gain of each such transistor may be controlled by a single element. The circuitry suggested in the above-noted patent is more expensive than that contemplated by the subject invention in that the matrixing of the color-difference signal and the luminance signal are accomplished in a separate stage and further, the color cathode ray tube is of a more expensive design in which distinct voltage adjustments are made to each of the CRT's guns to adjust its black level.